PALEONTOLOGIC INVESTIGATIONS OF THE PALEOZOIC ROCKS IN THE SIERRA LOS AJOS, NORTHERN SONORA, MEXICO

New paleontologic studies of Paleozoic rocks in the Sierra Los Ajos, northern Sonora, Mexico support stratigraphic correlation of these rocks to those of southern Arizona. Paleozoic rocks in the Sierra Los Ajos consist of the Cambrian Bolsa Quartzite and Abrigo Limestone, the Upper Devonian Martin Formation, and the Mississippian Escabrosa Limestone, and are lithologically and faunally similar to these same rock units in southern Arizona. The Paleozoic sequence in the Sierra Los Ajos represents the southernmost known exposure of cratonic rocks deposited along the southern margin of Paleozoic Laurentia. The Paleozoic rocks are underlain by the Lower Proterozoic Pinal Schist, and are overlain by non-marine sediments of the Upper Cretaceous Cabullona Group.

Trilobite collections (identified by A.R. Palmer) from the upper sandy member of the Abrigo Limestone represent two separate faunal horizons. A significant stage boundary occurs between these horizons, marking one of the most widespread extinction events in the Cambrian worldwide. This boundary correlates precisely to beds of the Abrigo Limestone in Cochise County, Arizona. The lower horizon belongs to the Crepicephalus Zone, the uppermost zone of the Marjumian Stage, and the higher horizon contains fauna of the Aphelaspis Zone, the lowermost zone of the Steptoean Stage of the Late Cambrian on Laurentia.

The Martin Formation in the Sierra Los Ajos area, like its counterpart in southern Arizona, is Late Devonian (primarily Frasnian) in age. Brachiopod-rich collections in the formation are dominated by mass accumulations of the late Frasnian Pseudoatrypa devoniana, a typical association for age equivalent Martin faunas in southern Arizona. Other fossils in the formation include stromatoporoids, Thamnopora, and rugose corals. The Mississippian Escabrosa Limestone forms the highest peaks of the Sierra Los Ajos. These rocks are typically crinoidal wackestone to packstone, and contain abundant macrofossils, including brachiopods, bryozoans, and colonial and solitary rugosans.